The most rapid phase of post-natal ocular growth occurs during the first years of life. Although much is known about the development of refractive error in infancy, there is little biometric data available on the ocular components during either infancy or early childhood. The course of infant refractive and component development can shed light on both basic mechanisms of eye growth as well as possible links with juvenile refractive error. Animal studies in chicken and primate models suggests that the eye is sensitive to the sign and magnitude of refractive error, with the eye adjusting its rate of growth to reduce these errors. Recent longitudinal studies of infant refraction have found associations between non- cycloplegic retinoscopy in infancy and in later childhood. Longitudinal refractive and biometric data from infancy will make it possible to examine the responsiveness of the eye to initial refractive error, the components responsible for emmetropization, and how component and refractive error, the components responsible for emmetropizatin, and how component and refractive development in the first years of life are related to ocular growth in childhood. The Berkely Infant Biometry Study (BIBS) is a five-year longitudinal study of the ocular components in infants and toddlers aged three months to three years. Its goal is to determine what changes occur in the eyes of young children during this time period and how these changes fit into a model of emmetropization that is either active or passive. Examination for component development in infants as a function of initial refractive error and as a function of the degree of emmetropization will indicate which components underlie ametropia and emmetropization. We measure refractive error, including astigmatism, corneal curvature in two meridians, anterior chamber depth, crystalline lens thickness and surface curvatures in two meridians, vitreous chamber depth, and axial length. We will determine how changes in the axial length, corneal curvature, and crystalline lens curvatures are coordinated during this rapid growth phase. We will investigate which ocular components-cornea or lens-are responsible for the previously reported astigmatism in infancy and its disappearance in early childhood.